Hi Pauline,
I've been looking at the same booking, and with CC it comes out at £901.00, which is for a motorhome 8.67m long, 3.2m high, sailing out on 30/07 and returning on 14/08 next year. It's actually cheaper, again with CC, to do St Malo both ways (£811.00) but I'm guessing you're the same as me and don't want to have to stay overnight nearby to get the morning crossing.
I did compare both options on the Brittany website, and they are around £80 more expensive. I don't know where you're staying, but we go to the Vendee, and a cheaper option we've used is Portsmouth to Le Havre, with LD Lines. Again cheaper with CC, £434.12 for next year, but you do have to pay it all up front with them. We found LD very good, they don't cram vehicles in like Brittany, and rooms and food are just as good.
Thanks, Lara.